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WASHINGTON (AP) -- Federal health regulators are weighing the risks of permitting Merck to sell its prescription respiratory pill, Singulair, as an over-the-counter medicine for allergies.

In a review posted online Wednesday, the Food and Drug Administration raised concerns that the drug could be used inappropriately by children or by patients with more serious conditions, like asthma.

The agency will ask a panel of advisers to vote on Friday on whether the drug is safe enough to be approved for nonprescription use.

A once-a-day pill, Singulair is used to relieve common allergy symptoms like congestion, runny nose, sneezing and watery eyes. First approved in 1997, Singulair was Merck's best-selling drug with revenue of $4.76 billion in 2011. But sales have plummeted since the company's patent expired mid-2012, exposing the brand to lower priced generic competition.

Merck & Co. Inc. hopes to give the drug a new life as an over-the-counter medication, competing alongside antihistamine pills like Claritin and nasal sprays like Nasacort.

But FDA regulators have questions about Merck's ability to scale back the former blockbuster medication to target adult patients suffering from allergies.

Currently Merck sells Singulair via prescription for both allergies and asthma in adults and children. An FDA analysis shows that about 38 percent of the 6.5 million to 7 million patients who use Singulair are younger than age 18.

Under Merck's proposal, the company would only market Singulair as an allergy medication for adults 18 and older. But the FDA said a study of whether patients understood the new label instructions "demonstrated that many teens selected to use the product, despite instructions not to use under age 18. This raises concern for inappropriate 'over-the-counter' use in children."

The FDA also notes that more than half of all prescriptions for Singulair today are for patients with asthma. Merck said it is not seeking over-the-counter approval for asthma because the condition can be life-threatening if not appropriately managed by a health care professional. But FDA reviewers note that switching the drug to an allergy-only indication "raises potential challenges" targeting allergy patients but not those with asthma.

The FDA will also ask its expert panel whether Merck's proposed drug label adequately warns patients of potential side effects. The drug's prescription label currently warns that patients have experienced agitation, hostility, depression, insomnia and restlessness while taking Singulair.

Singulair is part of a class of respiratory medications that work by blocking leukotriene, a chemical within the body that causes throat and nose airways to flare up and constrict.

The FDA has already approved the switch of several prescription allergy medications for over-the-counter use, most recently Sanofi's corticosteroid nasal spray, Nasacort. In 2011, the FDA approved the switch of Sanofi's antihistamine Allegra to nonprescription sales.

Merck says its polling shows suggests allergy sufferers could benefit from additional treatment options. According to company research, about 40 percent of customers currently using over-the-counter allergy medications switch between different products to get relief.

"We hope to provide for them another option," said George Philip of Merck's respiratory division, in an interview. "Singulair has shown efficacy and has demonstrated a safety profile that is appropriate for over-the-counter use."